5'ETS rRNA processing facilitated by four small RNAs: U14, E3, U17, and U3.

The nucleolus, the compartment in which the large ribosomal RNA precursor (pre-rRNA) is synthesized, processed through a series of nucleolytic cleavages and modifications into the mature 18S, 5.8S, and 28S rRNAs, and assembled with proteins to form ribosomal subunits, also contains many small nucleolar RNAs (snoRNAs). We present evidence that the first processing event in mouse rRNA maturation, cleavage within the 5' external transcribed spacer, is facilitated by at least four snoRNAs: U14, U17(E1), and E3, as well as U3. These snoRNAs do not augment this processing by directing 2'-O-methylation of the pre-rRNA. A macromolecular complex in which this 5'ETS processing occurs may then function in the processing of 18S rRNA.